1. Field of the Invention
This invention relates to a silicon nitride sintered body having high hardness and strength.
2. Related Art Statement
Recently, there have been various advanced attempts for using ceramics as structural materials for machines and the like, by utilizing the lightweight properties, thermal resistance, abrasion resistance and so on inherent to ceramics.
Particularly, it is known that when the ceramics are used as a bearing material, it is important to grasp the rolling fatigue life of the ceramic material. In order to enhance the rolling fatigue life, it is important to utilize materials having high strength.
Under such circumstances, there have hitherto been known a method of producing high density silicon nitride by converting a greater part of .alpha.-type silicon nitride (Si.sub.3 N.sub.4) into .beta.-type silicon nitride, and the like. These techniques aim to provide high strength silicon nitride products.
However, the inventors have made studies from all angles with respect to the bearing material and confirmed that it is favorable to include a given amount of .alpha.-type silicon nitride in the bearing material in order to enhance the rolling fatigue life among, as well as other properties required for use in the bearing material or the like. That is, it has been found that the hardness can be increased and the high strength can be maintained by including a given amount of .alpha.-type silicon nitride and the improved rolling fatigue life is first obtained by this synergistic action, and consequently the invention has been accomplished.